Digital knotmeter and log

ABSTRACT

A digital knotmeter and log for registering the speed of a boat in knots, and its elapsed distance in terms of the integral of its relative velocity through the water, in which the higher ordered digits of the measurement are independent of the velocity of sound in water and in which the lowest ordered digit is approximated by an internal counter.

l1 nite gtates atent Eek et al.

DKGITAL KNOTMETER AND LOG lnventors: Calvert F. Eek, Osterville; HowardH. Hill, Pocasset, both of Mass.

Assignee: Datamarine International Inc.,

Pocasset, Mass.

Filed: Mar. 8, 1971 Appl. No.: 121,869

10.8. Cl ..73/18l Int. Cl. ..G01c 21/10 Field of Search ..73/l8l, 194 A,290 V May 1, E973 [56] References Cited UNITED STATES PATENTS 3,329,0l 77/l967 Yamamoto et al. ..73/l94 A Primary ExaminerDonald O. WoodielAttorney-Rich & Ericson 5 7 ABSTRACT A digital knotmeter and log forregistering the speed of a boat in knots, and its elapsed distance interms of the integral of its relative velocity through the water, inwhich the higher ordered digits of the measurement are independent ofthe velocity of sound in water and in which the lowest ordered digit isapproximated by an internal counter.

10 Claims, 9 Drawing Figures /9 L TO DISPLAY llllll Patented May 1, 19733,729,993

5 Sheets-Sheet 1 2/ 22 T0 SENSQR 2 [355m Hm ON OFF LOG TO REMOTE Q KNOTSINDICATOR l i v I 0 F/G 5 SM 1 j I] OSCZHIIIIIIIII lllllll ll lllilll'//V VE N TORS CALVERT F. ECK

HOWARD H. HILL ATTORNEYS Patented May 1, 1973 3,729,993

5 Sheets-Sheet 2 HOWARD H. HILL ATTORNEYS Patented May 1, 1973 5Sheets-Sheet 4 W T m n W a 1 A o v x 21210:; 15m

CALVERT F. ECK

HOWARD H. HILL 7 ATTORNEYS Patented May 1, 1973 5 Sheets-Sheet 5 mamM/VE/VTORS CALVERT F. ECK

HOWARD H. HILL 5) 0 mm A Numo ATTORNEY-5 DIGITAL KNOTMETER AND LOG Ourinvention relates to the art of measuring the relative speed of a vesselin water, and to integrating its speed to determine distance traveled asa function of the measured velocity.

Navigation and piloting frequently require an indication of boat speedthat has been obtained in the past by numerous means, as by theconventional taffrail log, or by various devices which rely upon arotating member whose angular velocity is a function of the relativevelocity through the water. The taffrail log is notoriously inaccurate.Devices relying upon rotating vanes or impellers can be made reasonablyaccurate under fixed conditions of water temperature and salinity, butrequire a seal between the rotor and the internal parts which may bemore or less frictionally loaded in dependence on corrosion, silting,temperature and other variables. Moreover, unless constantly observed,such devices inevitably accumulate seaweed, algae, barnacles and otherdebris which change their calibration. Other speed measuring deviceshave been proposed in which the difference in propagation time betweenacoustic pulses travelling in opposite directions is measured as ameasure of the relative speed between an object and a fluid in which itis moving. Such a device is shown in U.S. Pat. No. 3,329,017 toYamamoto, et al. However, the relative speed of a vessel in water issuch that the time taken to complete an accurate measurement by priorapparatus of this type is too long for the purposes of practicalpiloting and navigation. The object of our invention is to facilitatethe rapid measurement of the relative velocity of a boat through thewater relatively independently of water salinity and temperature, andwith minimal interference from seaweed, silt, and other flotsam andjetsam.

The above and other objects of our invention are attained by an acoustictransceiver connected to a skeg affixed to the keel or hull of the boat.The skeg carrys a pair of spaced electro-acoustic transducers that arealigned with the fore and aft direction of the vessel. A timinggenerator is provided that basically establishes two fixed and equaltimes. During the first of these time intervals, one of the transducerssupplies acoustic pulses that travel through the water to the othertransducer at a speed greater or less than the speed of sound throughthe water by an amount determined by the relative speed of the boat withrespect to the water and the direction of motion of the vessel. Eachsuch pulse is used as a counting signal, and, when received by the othertransducer, nearly simultaneously re-excites the first transducer toproduce another pulse. This action continues throughout the first timingperiod. For each such transmitted pulse, a plurality of pulses isgenerated by an oscillator, and the sum of the transmitted pulses andthe oscillator pulses is accumulated in a register. During the secondtiming period established, the procedure is reversed, by which we meanthat the transducer formerly receiving acoustic pulses now serves as atransmitter to supply pulses to the other transducer through the water.These pulses, when received re-excite the sending transducer and whengenerated, trigger the oscillator to produce for each such pulse apredetermined number of following pulses. The transmitted pulses and theoscillator pulses are employed to step the counter in oppositedirections, such that the resulting contents of the register representsthe difference between the number of pulses transmitted in the directionof water flow and those transmitted against the direction of water flow.

It will be apparent that in general the sequence of oscillator pulsesproduced after each transmitted pulse will be interrupted at the end ofeach timing period in such a way that they will provide an approximatemeasure of the fraction of the time before another transmitted pulsewould have been produced. The net result is to approximate the nextlowest ordered digit of the speed indication produced by the relativedifference between the main pulses.

The timing apparatus establishes a third and fourth timing intervalfollowing each first and second interval. In the third such interval,the contents of the register are transferred to a digital speed displaywhich preferably indicates the speed of the vessel to within one-tenthof a knot. During the fourth timing interval, the contents of theregister are emptied into an accumulator through a divider circuit. Theaccumulator is provided with an indicator that registers the elapseddistance travelled by the vessel over as large a distance as it isdesired to provide for indication. The accumulator contents aredetermined as the approximate integral of the registered speed as afunction of time, in which the time increments correspond to theintervals between measurements.

The manner in which the apparatus of our invention is constructed, andits mode of operation, will best be understood in the light of thefollowing detailed description, together with the accompanying drawings,of a preferred embodiment thereof.

In the drawings:

FIG. 1 is a schematic diagram of a boat equipped with the speedmeasuring apparatus of our invention;

FIG. 2 is a schematic elevational view, on an en larged scale, showing aportion of the skeg and transducers in their relation to the hull of aboat equipped with the apparatus of our invention;

FIG. 3 is a schematic diagram of the indicator panel provided for theuse of the operator of the apparatus of our invention;

FIG. 4 comprises a schematic timing diagram illustrating various timingpulses controlling the apparatus of our invention;

FIG. 5 is a schematic diagram illustrating typical pulses occurringduring the operation of the apparatus of our invention;

FIG. 6 is a schematic block and wiring diagram of the speed and distancemeasuring apparatus of our invention in accordance with a specificembodiment thereof;

FIG. 7 is a schematic diagram of a transceiver forming a part of theapparatus of FIG. 6;

FIG. 8 is a schematic diagram of a count pulse generator forming a partof the apparatus of our invention; and

FIG. 9 is a schematic block and wiring diagram of a log control inaccordance with a particular embodiment of our invention.

Referring first to FIG. 1, we have shown a boat generally designated Ihaving a conventional rudder 2 ahead of which is mounted a propeller 3.A skeg 4 is mounted preferably well forward of the propeller 3 so as tobe as well isolated as possible from the acoustic noise generated by thepropeller. The skeg itself is best shown in FIG. 2, and, as indicated,is provided with fairings 5 and 6 interconnected by a bottom plate 7 anda top plate 8 all stream-lined to present minimum resistance to the flowof water past the hull. The skeg 4 is mounted to the hull 9 of the boatby meanshere shown as a through bolt 10 secured by a nut 11, and in theafter portion by means such as a screw 12. Cables 14 and 15 pass throughthe bolt 10 through sealed passages in the skeg 4 for connection toelectroacoustic transducers XA and X8, respectively, which are in turnembedded in sealed fairings l6 and 17, respectively. The parts of theskeg 4 and the fairings 16 and 17 may be of plastic, such as an epoxyresin or the like, preferably of any conventional variety so treated asto resist corrosion and the accumulation of marine life such as seaweed,algae, barnacles and the like. The electrical terminals of the crystalsXA and X8 are brought up through the associated cables 14 and 15 to ahousing 18 which contains electronic circuitry for sending and receivingpulses through the water, either by exciting the transducer XA such thata pulse will be received at XB, or by exciting the transducer XB so thata pulse will be received by the transducer XA. The housing 18 and theelectronics contained therein are preferably designed for short leadsand short time constants such that the electronic propagation timebetween a signal received at one of the transducers such as X8 and therepropagated signal responsive to that received signal transmitted tothe other transducer such as XA is minimized. In addition, care shouldbe taken to minimize the difference in the time in which a signalreceived at X8 is acknowledged at XA, and viceversa. A cable 19 leads toa display unit; the propagation time through this cable is not critical.

Referring to FIG. 3, the cable 19 leads to a display indicator generallydesignated 20 provided with an on/off switch S1, a three-digit knotindicator 2], and a decimal log indicator. As here shown, the log isprovided with four digit indications ahead of the decimal point and onedigital indication after the decimal point, so that it can produce amaximum indication in knots of 9999.9. The indicator 21 can produce adecimal indication to within one-tenth of a knot that will normally notexceed, for example, 40 knots, although, of course, its capacity isgreater than that.

The indicator unit 20 is provided with a cable 23, leading to thevessels power supply in a manner that will appear in somewhat moredetail below, and, if desired, with a cable 24 leading to one or moreremote indicators which may be similar to the unit 20, or may simplyinclude the on/off switch and knots indicator features, as in onepractical embodiment of our invention.

The apparatus of our invention basically functions on a timing cyclethat is indicated in FIG. 4. By conventional means, a timing pulse A isgenerated which is followed after a brief interval by a timing pulse Bof exactly the same duration as the timing pulse A. These pulses may beproduced in a conventional manner by a crystal controlled oscillatorthat will be discussed in somewhat more detail below. By similarconventional means, a timing pulse C is provided following the pulse Bby a brief interval, and a timing pulse D is provided following thetiming pulse C. The cycle is then repeated with another A pulse. Byconventional means, which it is considered unnecessary to describe indetail, these timing pulses are inverted where required, and in thoseinstances are shown and described as A, B G and 5, respectively.

Referring next to FIG. 5, the A pulse has been elongated to indicate atypical sequence of operations. During the A pulse, the transducer XA isselected and excited with a pulse SAR that is propagated toward thetransducer XB with a velocity that will be assumed to be V V where V isthe speed of sound, assuming water flow V in a direction indicated bythe arrow in FIG. 2. As will appear, receipt of this pulse by XB willcause XA to emit another pulse, so that a second SAR pulse will beproduced when the pulse is received by the transducer XB. Typically, thetransducers XA and X8 may be spaced so that propagation time betweenthem in still water is about microseconds.

By means to be described, following each SAR pulse there is internallygenerated a sequence of nine pulses labelled OSC2 in FIG. 5 at such arate that all nine will be received before the next SAR pulse isproduced, in either direction of propagation, under the most extremeconditions of salinity and temperature to be encountered. As indicatedin FIG. 5, in general the end of the A pulse will occur at some pointduring the propagation of the nine OSC2 pulses following the last SARpulse. The total number of SAR and OSC2 pulses thus produced during an Apulse, to be labelled hereinafter as GEN, is thus at least approximatelyequal to the number of SAR pulses received to the nearest one-tenth ofan SAR pulse.

As will appear, during the B interval shown in FIG. 4, exactly the sameoperation takes place as indicated in FIG. 5, except that in this casethe transducer XB acts as the pulse receiver. In this case, thetransmitted pulses are propagated through the water at a speed V V sothat the total count during a B pulse will be less than the total countproduced during an A pulse, sup posing the boat, as in FIG. 1, to bemoving forward in the water.

As will appear, the pulses produced during the A time are supplied to anaccumulator constituting a reversible counter, and each pulse producedduring the succeeding 8 time is subtracted from the accumulator so thatat the end of an A pulse and a B pulse the contents of the accumulatorrepresent the speed of the boat through the water. It should be noted inthis regard that, because of the residual contents of the accumulatorrepresent the difference between the number of pulses transmitted inopposite directions, it is immaterial whether the SAR pulses are countedas they are transmitted, or as they are received. The independance ofthe pulse difference so detected on water temperature salinity can beshown mathematically as follows:

Let the transit time in the direction of the flow be given by 1,, wherein which d is the effective distance between the transducers, V is thespeed of the boat, and V, is the speed of sound in water under theprevailing conditions. Then 1 the transit time in the oppositedirection, will be given by If the equal times A and B are expressed asT, then the number N1 of SAR pulses transmitted in A time is given byand the number N2 of SAR pulses transmitted in B time is given by Let NN, N where N is the difference in the number of SAR pulses transmittedin A time and B time. Then N (2TVa)/d; that is, the accumulated numberof counts is independent of the speed of sound in water.

Referring next to FIG. 6, we have shown the apparatus of our inventionin somewhat more detail, with certain elements shown in block diagramform. As indicated, power is supplied by a conventional battery B and iseffective to cause operation of the apparatus when the switch S1, shownalso in FIG. 3, is closed. A potential B of for example 12 volts, isthereby produced. This voltage is supplied to a conventional regulatedpower supply 25 that produces a pair of regulated voltages of 8 of, forexample, of volts.

The potential 13 is also supplied to a brightness control hereschematically shown as a potentiometer P1 that supplies an adjustableilluminating voltage 8 to a series of decimal indicating tubes HKl, TKl,and UKl, which indicate the tens, units, and tenths units respectively,of the registered speed in a manner to ap pear. Each of these tubes isprovided with seven conventional filaments Fl through F7, as shown forthe tube HKl. These filaments are arranged in a rectangular conventionalarray, as better suggested schematically for the tubes TKl and UKI, tobe selectively illuminated in a manner familiar to those skilled in theart so that decimal units from zero to nine can be displayed byselective energization of the filaments.

As indicated, for the tube HKI, any particular filament can be energizedby applying a ground level current sink to its associated input terminala through g. The apparatus for providing appropriate energizing signalsto these indicating tubes, which it will be appreciated are locatedbehind the panel 21 in FIG. 3, will next be described.

Basic timing for the apparatus is provided by a crystal controlledoscillator OSCl, which may be of, for example, 200 HZ in Frequency, andwhich is preferably adjustable for calibration purposes. In particular,since the speed indication provided is determined by the differencebetween the number of pulses propagated during A time and the numberpropagated during B time, the oscillator OSCl should be adjusted so thatthe resulting difference will be a direct measure of the speed in thedesired units; i.e., knots, m.p.h., etc.

The output signal, labelled OSCI, is employed to actuate the log forminga part of our invention, as will be described, andis also directlyconnected to a conventional timing pulse generator that produces thepulses A, B, C and D described above, on independent leads, and in thesequence shown in FIG. 4, by conventional pulse dividing the gatingcircuits which it is though unnecessary to describe in detail. The powersupply potential 8;, is supplied to the timing pulse generator such thateach of the pulse signals A, B, 3, and O is at B or 5 volts, when it ispresent, and is at 0 volts, or ground potential, when it is not present.These polarities are, of course, a matter of choice or design, but areincluded to facilitate an understanding of the invention.

The apparatus further includes a transceiver generally designated 26contained within the housing 18 in FIG. 2 and connected to thetransducers XA and X8 in a manner to be described below such that SARpulses are produced during the time period A, in response to pulsespropagated from XA to XB. Similar pulses, SAR are produced during theperiod B when the pulses are propagated from XB to XA.

These pulses SAR are supplied to a count pulse generator generallydesignated 27. The count pulse generator 27 is also supplied with apower supply voltage 8 and a STOP signal labelled STOP from a counter 28in response to each ninth pulse OSC2 produced by the count pulsegenerator 27 in a manner to be described.

The count pulse generator produces an output signal labelled GEN andcomprising pulse trains of the type shown in FIG. 5 which include boththe pulses SAR and the following pulses OSC2. These pulses GEN areapplied to a reversible binary coded decimal counter generallydesignated 29 and comprising a 4 bit reversible unit counter stage UC, a4 bit tens counter stage TC, and a 4 bit hundreds counter stage HC. Thecounter stage UC is connected to the counter stage TC in a conventionalmanner by a carry line CU and a borrow line BU. Similarly, the tensstage TC is connected to the hundreds stage HC by carry line CT andborrow line BT. These circuits may be conventional and they need not bedescribed in further detail.

The counter 29 is stepped up or down by logic 0 pulses applied either toits up or down lines. During the A time, pulses GEN are supplied througha NAND gate 31 to cause the counter 29 to count up. During B time, thepulses GEN are supplied through an AND gate 32 followed by a NOR gate 33to cause the counter to step down. The preferred frequency of countingis such that somewhat more than 14,000 GEN pulses will be supplied tothe counter during A time and somewhat less than 14,000 pulses will besupplied during B time. The counter will obviously overflow a number oftimes during each A and B pulse. However, in accordance with ourinvention, the number to be measured, which is the difference betweenthe pulses supplied during the A time and the B time, will always be inthe last three digits. Thus, it is not necessary to register or takeaccount of these overflow conditions.

The binary coded decimal outputs of the counter stages TC, UC and HClabelled, for example, H1, H2, H3, and H4 for the counter stage HC, arestrobed at T4 time into a 4 bit tens register HR, :1 4 bit unitsregister TR, and a 4 bit tenths register UR through gates labelled HG,TG and UG, respectively. As indicated, these gates may be simply ANDgates of the type shown at 34 in the unit HG. Once stored in theregisters, HR,

TR, and UR, the signals, which now will be the difference in countbetween the last preceding A and 13 pulses, are supplied throughconventional binarycoded-decimal-to-7-line encoders l-iE, TE and UE, tosupply appropriate energizing signals to the indicating lamps HKl, TKland UKll, to indicate the tens, units, and tenths digits of the speed ofthe vessel in knots.

During the time pulse D, the contents of the counter 29 areprogressively reduced to zero by pulses from the oscillator OSClsupplied through an AND gate 35 enabled by the pulse D as one input to aNOR gate 33 to count down the counter until a zero detector generallydesignated 36 indicates that the contents of each of the registers UC,TC and HC has been reduced to zero. The count down pulses CD from thezero detector 36 are supplied both to the AND gate 35 and to a counter37. The counter 37 divides the number of pulses by 7200 and supplies anoverflow pulse through an amplifier generally designated 38 to amechanically actuated digital log indicator generally designated 39 thatdisplays the accumulated integral of the measured speed of the craft inknots, or, if desired, in statute miles or other convenient units. Thenumber 7200 is selected in terms of the approximation to the integral fv22; that is determined by the increments At between the measurements ofv so that the output represents the elapsed distance D in nauticalmiles, or other desired units.

FIG. 7 shows the details of the transceiver 26 in accordance with apreferred embodiment of the invention. The transceiver 26 is normallydisabled by an npn transistor Q1 that has its base connected to receivethe signal A through a diode CR1, and, in parallel, to receive thesignal B through a diode CR2. When both of these signals is absent,there is a +5 volt potential applied to block the diodes CR1 and CR2 andthereby allow the transistor Q1 to be forward biased through a resistorR1 connected between its base and the supply voltage B2+. That shunts acapacitor C1 in a unijunction transistor circuit comprising aunijunction transistor Q2 having its emitter connected between oneterminal of the capacitor C1 and ground. A timing resistor R2 isconnected between the emitter of the transistor Q2 and the supplyterminal B2+.

One base of the transistor Q2 is returned to the supply terminalBZ-ithrough a resistor R3, and its second base is returned to groundthrough a resistor R4.

It will be apparent that when either of the timing signals A or B ispresent at ground potential, a ground level sink is supplied through thediode CR1 or CR2 to cut off the transistor 01 and allow the capacitor C1to charge through the resistor R2. The unijunction transistor Q2 willthereby be biased into conduction, producing a pulse across the resistorR4 that is initially positive going and decays to a lower constantpositive value, to present a pulse signal allowing the selected one ofthe piezo-electric crystals XA or XB to be excited in a manner that willnext be described.

The input signal A is applied through a resistor R to the base of an npntransistor Q3 that has its emitter grounded. The collector of thetransistor Q3 is returned to 82+ through a resistor R6, and is connectedto ground through a capacitor C3. The collector of the transistor Q3 isalso connected to the emitter of a pnp transistor Q4 that has itscollector connected to a circuit extending from the collector of thetransistor Q4 through the piezo-electric crystal XA to ground, and inparallel with the crystal, through an inductor L1 and protective diodeCR7. A first circuit is coupled to the base of the transistor Q4 fromthe second base of the unijunction transistor Q2 through a diode CR3.The second base of the transistor Q2 is also connected through the diodeCR3 and a resistor R5 to the supply terminal B2+. A second controlcircuit for the base of the transistor Q4 extends from the upperterminal of a parallel circuit including the piezo-electric crystal XB,an inductor L2, and a protective diode CR8. This circuit extends fromthe upper terminal of the inductor L2 through a resistor R9 to the baseof the transistor Q4.

Transistors Q5 and 06 are connected in a circuit essentially symmetricalwith that comprising the transistors Q3 and Q4 for the control of thecrystal XB. As shown, when the signal B is absent, a positive voltage issupplied through the resistor R11 to the base of an npn transistor Q5,biasing it into conduction and bringing the emitter of a pnp transistorQ6 essentially to ground potential. The emitter of the transistor Q6 isconnected to the positive supply terminal B2+ through a resistor R8, andto ground through a storage capacitor C. The collector of the transistorQ6 is connected to ground through the parallel circuit comprising thecrystal XB, the inductor L2 and the protective diode CR8.

The collector of the transistor 06 is also connected to the base of thetransistor Q4 through a resistor R9. The base of the transistor Q6 isconnected to the collector of the transistor Q4 through a resistor R10,and is connected through a resistor R7 to the supply terminal B2+. Thebase of the transistor O6 is also returned to the second base of theunijunction transistor 02 through a diode CR4.

An output amplifier circuit is provided comprising an npn transistor Q7having a grounded emitter and a collector returned to the supplyterminal B2+ through a resistor R13. The base of the transistor Q7 isconnected to 82+ through a resistor R12, and is connected to the anodesof a pair of diodes CR5 and CR6 through a resistor R14. The diodes CR5and CR6 form an OR gate for pulsing the amplifier comprising thetransistor Q7, as will appear.

In the presence of a negative-going current sink applied to the cathodeof either of the diodes CR5 or CR6, the potential of the base of thetransistor Q7 is reduced. That cuts off the transistor Q7 and produces apositive-going pulse SAR. The manner in which this pulse is producedduring the operation of the apparatus will next be considered.

First, assuming that the pulse A is present at ground level to drawcurrent through the resistor R1 and the diode CRl, cutting 01? thetransistor Q1. As the same time, the ground level signal A will beapplied through the resistor R15 to cut off the transistor Q3. Prior tothe application of the ground level pulse A, capacitor C3 is by-passedby current flowing through the resistor R6,

the collector-to-emitter path of the transistor Q3, and through theinductor L1 to ground. In the presence of the pulse A, the transistor Q3is cut off and the capacitor C3 will be charged.

With the transistor Q1 now cut off, the capacitor C1 will be chargedthrough the resistor R2, and the emitter potential of the unijunctiontransistor Q2 will rise to the point at which the unijunction willdischarge and produce a positive pulse across the resistor R4. Whilethis charging of the capacitor C1 is taking place, current flowing fromthe potential B2+ through the resistor R6 in the emitter-to-collectorpath of the slightly forward biased transistor Q4 will flow through theinductor L1.

When the unijunction transistor Q2 does discharge, the positive pulseproduced across the resistor R4 will be coupled through the diode CR3 tocut off the transistor Q4 and abruptly interrupt the current through theinductor Ll. A sharp negative-going pulse will thereby be applied to thetransducer XA, causing a corresponding acoustic pulse to be propagatedtowards the transducer XB at the velocity of sound in the water underprevailing conditions plus the velocity of the boat in the direction ofmotion. This pulse will be severely damped in the positive direction bythe diode CR7, so that a minimum of ringing will occur.

When this negative signal from the transducer XA, attenuated by some 20or 30 db, is received by the transducer XB, an inverted andpositive-going pulse will be produced across the inductor L2 that willbe coupled to the base of the transistor Q4 through the resistor R9,causing it to be momentarily cut off again. Cutting off the transistorQ4 will again interrupt conduction through the inductor L1 and produceanother negative pulse across the transducer XA, propagated toward thetransducer XB and actuating the amplifier Q4 to produce a second pulseSAR.

In the interval between the pulses A and B, the transistor Q1 will againbe gated into conduction and discharge the capacitor C1. When the pulseB appears at ground potential, the capacitor C1 will again change to thefiring potential of the unijunction Q2, producing another positive spikedecaying to a positive level across the resistor R4. The spike in thiscase will cut off the transistor Q6, through the diode CR4. In thiscase, the current through the inductor L2 will be abruptly interrupted,propagating a negative pulse toward the transducer XA against thedirection of water flow. At the same time, the potential at the cathodeof the diode CR6 will be dropped, turning off the transistor Q7 andproducing an SAR pulse. The interval between SAR pulses produced duringthe B pulse will evidently be longer than the interval between thoseproduced during the A interval, because of the relative direction ofwater flow.

Referring now to FIG. 8, the details of the count pulse generator inaccordance with the preferred embodiment of the invention are shown. Theapparatus may comprise a conventional synchronous digital J-K flip-flopFl which may be assumed to be provided with conventional gated set andreset terminals S and R which respond to an applied logic level when anegative-going clock pulse is applied to a trigger terminal C, anddirect set and direct reset input terminals DS and DR which respond toapplied logic 0 signal levels to direct the state of the flip-flopindependently of the trigger terminal. As shown, the terminal R of theflipflop F1 is grounded, so that a pulse applied to the clock pulseterminal C will set the flip-flop. When set, the

logic 0 output terminal of the flip-flop is at 0 volts, and the logic 1output terminal is at logic 1, or positive. The flip-flop Fl willaccordingly produce a ground level signal at its logic 0 output terminalwhen set by the negative-going trailing edge of an input pulse SAR fromthe transceiver described above in connection with FIG. 7. When reset,the flip-flop Fl. will apply a positive signal to a resistor RM to biasa transistor Q8 of the npn variety into conduction. The transistor Q8has its emitter grounded, and its collector connected to the positivesupply terminal B2+ through a calibration potentiometer P2 and a fixedresistor R17 in series. The resistor P2 maybe used to calibrate theapparatus for both salinity or temperature or both. Its function will bedescribed in more detail below.

The apparatus 27 comprises a relaxation oscillator formed by aunijunction transistor Q9 having a first base returned to the supplyterminal B2+ to a resistor R18 and a second base returned to groundthrough a resistor R19. The emitter of the unijunction transistor Q9 isconnected between the junction of the potentiometer P2 and a capacitorC4. The capacitor C4 is connected between the emitter of the transistorQ9 and ground, and is also connected between the collector and theemitter of the transistor Q8. It will be apparent that when theflip-flop F1 is set, the logic 0 potential appearing at the 0 terminalof the flip-flop F 1 will cut off the transistor Q8 and allow thecapacitor C4 to be charged, through the resistor R17 and that portion ofthe potentiometer P2 selected by its setting, until the firing potentialof the transistor O9 is reached, at which point the capacitor C4 will bedischarged through the emitter-to-base junction of the transistor Q9 andthrough the resistor R19 to ground. That will produce a positive OSC2pulse that is applied to step up a divideby-lO counter 44, of anyconventional binary variety, and to one input terminal of a NOR gate 43.

The counter 44 is provided with two output terminals labelled X and Zwhich may comprise the logic 1 values of the 2 digit and the 2 digit,respectively, of the counter states, representing the count 9. At thiscount, the signals X and Z are applied as positive signals to two inputterminals of a NAND gate 42. A third input terminal of the NAND gate 42receives the output of a NAND gate 41 which inverts the received pulseSAR and thereby produces a positive input signal when SAR is absent. Thecombination of the signals SAR absent and X and Z present will thusproduce a logic 0 at the output terminal of the gate 42, resetting theflip-flop F l and turning off the relaxation oscillator comprising theunijunction transistor Q9. The'result will be 9 OSC2 pulses applied tothe NOR gate 43, each of which will produce a GEN pulse. The SAR pulsewhich initially set the flip-flop F1 and triggered the train of 9 OSC2pulses is also applied to the NOR gate 43 to produce a GEN pulse,such'that a train of 9 pulses following each SAR pulse, with thepossible exception of the last, is produced, as shown in H6. 5. Thesepulses are applied to step the counter 29 in FIG. 6 up or down asdetermined by the prevailing interval time A or B, respectively.

Referring next to FIG. 9, the details of the zero detector 36 are shown.Basically, this detector comprises three NOR gates 44, 45 and 46 whichreceive the BCD outputs of the counter stages l-iC, TC and UC of the lllcounter 29. As long as any one of these signals H1 through U4 is atlogic 1, the output of the corresponding gate 44, 45, or 46 will be atlogic 0, disabling a NAND gate 47 which receives the outputs of thegates 44 45 and 46. Under these conditions, the gate 47 will produce alogic 1 output signal, enabling a NAND gate 43 that receives thepositive pulses OSCl and D. The output of the NAND gate 48 producesnegative going pulses in response to OSCl while enabled by the 1 fromzero detection gate 47 and time interval D. Gate 49 inverts the pulsesfrom gate 48 to provide positive going pulses.

These pulses are applied to gate 35 along with time interval D to countdown the accumulator to zero. These pulses are also supplied to the 7200counter 37 to advance the log. Thus, the gate 35 counts down thecontents of the counter 29 until a count is detected. Each count downpulse applied to the gate 35 by the gate 49 is also applied to thedividing counter 37, which overflows at each 7200th count to produce anoverflow signal applied through an amplifier 38 to actuate a relay K1and thereby step a mechanical digital log indicator 39 of anyconventional construction.

Overall operation of the apparatus of our invention will be apparentfrom the general description of the system in FIG. 6 and the detaileddescription of the several components with respect to FIGS. 7-9.Briefly, when the switch S1 is closed, the transceiver 26 willalternately supply approximately 1400 pulses from the crystal XA to thecrystal XB, resulting in SAR pulses that step up the counter 29 duringthe time interval A. During the time interval B, a lesser number ofpulses, still approximately in the vicinity of 1400, will be appliedfrom the transducer XB to the transducer XA, producing somewhat lessthan 1400 SAR pulses that are applied to step down the counter 29.Following each received pulse SAR in either the count up or count downmode of operation. nine additional pulses are produced by the countpulse generator 27, so that for each received pulse SAR, except perhapsthe last, during each interval A and B, there will be 10 pulses counted.Since the difference between the count up and count down sequence willalways be less than 1000 counts, overflow of the counter 29, (HO. 6) isimmaterial, as the residual difference will reflect the actual speed. Itwill be apparent that the accuracy of the instrument will depend uponhow nearly the difference between the eighth OSC2 pulse produced by thegenerator of FIG. 8 and the ninth equals the difference between theninth pulse and the next SAR pulse. That can be controlled in dependenceupon salinity and temperature so that considerable accuracy can beprovided under any particular set of conditions. More importantly, itshould be noted that the setting of the potentiometer P2, which controlsthe interval between the pulses OSC2, could be adjusted to reflect thenormal conditions under which the particular vessel will be operated. Anadditional feature of the invention is that even though the exact tenthsreading represented by the knotmeter may be affected both by internalelectronic propagation time and by local variations in salinity andtemperature, the direction of increase or decrease in speed is aninvaluable aid in trimming the boat to take advantage of prevailingconditions. In particular, in trimming the sails of a sailboat, if anadjustmerit results in an increased indication of one-tenth ortwo-tenths knots, it will be obviously an improvement even if the exactspeed reading is off by more than the change in the indication.

While we have described our invention with respect to the details of apreferred embodiment thereof, many changes and variations will beapparent to those skilled in the art upon reading our description, andsuch can obviously be made without departing from the scope of ourinvention.

Having thus described our invention, what we claim is:

1. Apparatus for measuring the velocity of an object relative to a fluidin which the object is at least partially immersed comprising apair ofconfronting transducers coupled to said object along an axis,

means for exciting a first of the transducers to propagate signals alongsaid axis toward the second for a predetermined time interval,

means for producing a predetermined number of signals following eachpropagated signal,

reversible accumulator means for accumulating said propagated andproduced signals during said time interval,

means for exciting said second transducer to propagate signals towardsaid first transducer for a second time interval equal to said firsttime interval,

means responsive to each signal propagated toward said first transducerfor producing said predetermined number of signals, and

means for decrementing the contents of said accumulator once for eachsignal propagated and produced during said second time interval, wherebythe residual contents of said accumulator means represents the relativespeed of the object along said axis through the fluid.

2. The apparatus of claim 1, including means actuated following each ofsaid second time intervals for indicating the contents of saidaccumulator.

3. The apparatus of claim 1, further comprising means for accumulatingthe residual contents of said accumulator over a sequence of said firstand second time intervals to provide a measure of distance elapsed bythe object through the fluid.

4. A marine speedometer, comprising a skeg mounted on the hull of avessel beneath the waterline thereof and aligned parallel to the keel ofthe vessel,

a pair of transducers mounted in spaced confronting relationship on saidskeg and in alignment therewith,

singaround circuit means connected to said transducers and responsive toa first applied signal for propagating a first series of signals from afirst of said transducers toward the second at intervals determined bythe propagation time of said signals from the first transducer to thesecond during said first applied signal, said singaround circuit meansbeing responsive to a second applied signal for propagating a secondseries of signals from said second transducer toward said firsttransducer at intervals determined by the propagation time of saidsignals from the said second transducer to said first transducer duringsaid second applied signal,

means responsive to said first applied signal and said first propagationsignals for generating a train of a predetermined number of signalsfollowing each first signal propagated between said transducers,

means responsive to said second applied signal and said secondpropagated signals for generating a train of said predetermined numberof signals following each second signal propagated between saidtransducers,

means for alternately applying said first and said second signal to saidsingaround circuit means for equal time durations,

and reversible accumulator means connected to said singaround circuitmeans and responsive to said first applied signal for registering thenumber of propagated and generated signals during said first appliedsignal, said accumulator means being responsive to said second appliedsignal for decrementing the contents of said accumulator means once foreach propagated and generated signal during said second applied signal.

5. The apparatus of claim 4, further comprising a register,

means actuated following each second signal for transferring thecontents of said accumulator to said register, and

indicating means coupled to said register for indicating the speed ofthe vessel in terms of the contents of said register.

6. The apparatus of claim 5, further comprising a second register, and

means effective following the transfer of the contents of saidaccumulator to said first register to add the contents of saidaccumulator to said second register, thereby providing a measure ofelapsed distance,

7. The apparatus of claim 6, in which said addiing means comprises asignal divider to cause the contents of said second register torepresent the elapsed distance in predetermined units.

8. In combination with an object at least partially immersed in a fluidand an acoustic flowmeter mounted on said object, said flowmeter beingof the type comprising a reversible accumulator, singaround transceivermeans coupled to said accumulator for producing signals at a first ratedetermined by the rate of propagation of sonic signals through the fluidfrom a first transducer on the object to a second transducer on theobject and at a second rate determined by the rate of propagation ofsonic signals from said second transducer to said first transducer, saidsignals being propagated for equal times in said opposite directions andadded in the accumulator while propagated in one direction andsubtracted from the accumulator while propagated in the oppositedirection, that improvement comprising means for producing a traincomprising an equal number of signals following each propagated signaland preceding the next, and means for adding or subtracting thoseproduced signals following propagated signals to or from saidaccumulator during said equal times according as said propagated signalsare added or subtracted, respectively whereby the contents of saidaccumulator following a pair of said e ual times represents the relativevelocity of said ob ec in said fluid along the line between saidtransducers more accurately than would the difference between thesignals actually propagated during said equal time intervals.

9. ln combination with an acoustic flowmeter comprising a singaroundcircuit producing a first series of signals propagated at a first ratein a first direction in a fluid in which the flowmeter is at leastpartially immersed, each succeeding said signal being responsive to itspreceding signal, and a second series of signals propagated at a secondrate in a second sense in a direction opposite said first direction insaid fluid, in which said first and second rates are determined by therelative velocity of the flowmeter in the fluid along a line parallel tosaid opposite directions,

relaxation oscillator responsive to each signal propagated through saidmedium to produce a signal train,

a counter controlled by said signal train to disable said oscillatorprior to the shortest time between propagated pulses,

an accumulator,

means for incrementing said accumulator for a first predetermined timein response to each of said first propagated signals and each signalproduced by said oscillator during said first time, and

means for decrementing said accumulator for a second time equal to saidfirst time in response to each of said second propagated signals andeach signal produced by said oscillator during said second time.

10. The apparatus of claim 9, further comprising a register,

a speed indicator controlled by the contents of said register, and

means for copying the contents of said accumulator into said register atthe end of each second time.

1. Apparatus for measuring the velocity of an object relative to a fluidin which the object is at least partially immersed comprising a pair ofconfronting transducers coupled to said object along an axis, means forexciting a first of the transducers to propagate signals along said axistoward the second for a predetermined time interval, means for producinga predetermined number of signals following each propagated signal,reversible accumulator means for accumulating said propagated andproduced signals during said time interval, means for exciting saidseconD transducer to propagate signals toward said first transducer fora second time interval equal to said first time interval, meansresponsive to each signal propagated toward said first transducer forproducing said predetermined number of signals, and means fordecrementing the contents of said accumulator once for each signalpropagated and produced during said second time interval, whereby theresidual contents of said accumulator means represents the relativespeed of the object along said axis through the fluid.
 2. The apparatusof claim 1, including means actuated following each of said second timeintervals for indicating the contents of said accumulator.
 3. Theapparatus of claim 1, further comprising means for accumulating theresidual contents of said accumulator over a sequence of said first andsecond time intervals to provide a measure of distance elapsed by theobject through the fluid.
 4. A marine speedometer, comprising a skegmounted on the hull of a vessel beneath the waterline thereof andaligned parallel to the keel of the vessel, a pair of transducersmounted in spaced confronting relationship on said skeg and in alignmenttherewith, singaround circuit means connected to said transducers andresponsive to a first applied signal for propagating a first series ofsignals from a first of said transducers toward the second at intervalsdetermined by the propagation time of said signals from the firsttransducer to the second during said first applied signal, saidsingaround circuit means being responsive to a second applied signal forpropagating a second series of signals from said second transducertoward said first transducer at intervals determined by the propagationtime of said signals from the said second transducer to said firsttransducer during said second applied signal, means responsive to saidfirst applied signal and said first propagation signals for generating atrain of a predetermined number of signals following each first signalpropagated between said transducers, means responsive to said secondapplied signal and said second propagated signals for generating a trainof said predetermined number of signals following each second signalpropagated between said transducers, means for alternately applying saidfirst and said second signal to said singaround circuit means for equaltime durations, and reversible accumulator means connected to saidsingaround circuit means and responsive to said first applied signal forregistering the number of propagated and generated signals during saidfirst applied signal, said accumulator means being responsive to saidsecond applied signal for decrementing the contents of said accumulatormeans once for each propagated and generated signal during said secondapplied signal.
 5. The apparatus of claim 4, further comprising aregister, means actuated following each second signal for transferringthe contents of said accumulator to said register, and indicating meanscoupled to said register for indicating the speed of the vessel in termsof the contents of said register.
 6. The apparatus of claim 5, furthercomprising a second register, and means effective following the transferof the contents of said accumulator to said first register to add thecontents of said accumulator to said second register, thereby providinga measure of elapsed distance,
 7. The apparatus of claim 6, in whichsaid addiing means comprises a signal divider to cause the contents ofsaid second register to represent the elapsed distance in predeterminedunits.
 8. In combination with an object at least partially immersed in afluid and an acoustic flowmeter mounted on said object, said flowmeterbeing of the type comprising a reversible accumulator, singaroundtransceiver means coupled to said accumulator for producing signals at afirst rate determined by the rate of propagation of sonic signalsthrough the fluid from a first Transducer on the object to a secondtransducer on the object and at a second rate determined by the rate ofpropagation of sonic signals from said second transducer to said firsttransducer, said signals being propagated for equal times in saidopposite directions and added in the accumulator while propagated in onedirection and subtracted from the accumulator while propagated in theopposite direction, that improvement comprising means for producing atrain comprising an equal number of signals following each propagatedsignal and proceding the next, and means for adding or subtracting thoseproduced signals following propagated signals to or from saidaccumulator during said equal times according as said propagated signalsare added or subtracted, respectively whereby the contents of saidaccumulator following a pair of said equal times represents the relativevelocity of said object in said fluid along the line between saidtransducers more accurately than would the difference between thesignals actually propagated during said equal time intervals.
 9. Incombination with an acoustic flowmeter comprising a singaround circuitproducing a first series of signals propagated at a first rate in afirst direction in a fluid in which the flowmeter is at least partiallyimmersed, each succeeding said signal being responsive to its precedingsignal, and a second series of signals propagated at a second rate in asecond sense in a direction opposite said first direction in said fluid,in which said first and second rates are determined by the relativevelocity of the flowmeter in the fluid along a line parallel to saidopposite directions, a relaxation oscillator responsive to each signalpropagated through said medium to produce a signal train, a countercontrolled by said signal train to disable said oscillator prior to theshortest time between propagated pulses, an accumulator, means forincrementing said accumulator for a first predetermined time in responseto each of said first propagated signals and each signal produced bysaid oscillator during said first time, and means for decrementing saidaccumulator for a second time equal to said first time in response toeach of said second propagated signals and each signal produced by saidoscillator during said second time.
 10. The apparatus of claim 9,further comprising a register, a speed indicator controlled by thecontents of said register, and means for copying the contents of saidaccumulator into said register at the end of each second time.